1. Field of the Invention
The present invention relates to a method of transmitting/receiving data in a sensor node for reducing overhearing of sensor nodes, and a sensor network which implements the method.
The present invention is derived from a research project supported by the Information Technology (IT) Research & Development (R&D) program of the Korean Ministry of Information and Communication (MIC) and the Institute for Information Technology Advancement (IITA) [Subproject NO.: 2005-S-106-03, Subproject Name: Development of Sensor Tag and Sensor Node Technologies for RFID/USN].
2. Description of the Related Art
When a node which desires to transmit data in a sensor network that operates in an asynchronous manner, especially in a media access control (MAC) layer of the sensor network, notifies neighboring nodes that it has data to be transmitted (or transmission data), or transmits the data, the neighboring nodes may receive a signal generated from the notification or transmission even if the signal is not needed by the neighboring nodes. Such a phenomenon is referred to as overhearing. A sensor node that overhears an unwanted signal consumes power, i.e., energy due to the reception of the signal. There are several conventional techniques for reducing unnecessary energy consumptions.
First, the Berkeley MAC, also called the B-MAC (J. Polastre & J. Hill & D. Culler, ‘Versatile Low Power Media Access for Wireless Sensor Networks’, ‘Proceedings of the 2nd International Conference on Embedded Networked Sensor Systems’, pp. 95-107, November 2004) reduces the overhead of a protocol for synchronization by controlling a network to operate in an asynchronous manner by using preamble sampling, and reduces energy consumption by minimizing an awake operation in which each node checks whether its neighboring nodes have transmission data destined for itself in an environment where no data transmission is performed.
FIG. 1 is a diagram for explaining operations of sender node 101, receiver node 102, and other nodes 103, according to the B-MAC.
Referring to FIG. 1, the nodes 101, 102, and 103 perform a low power listening (LPL) operation in which they wake up at different times at predetermined intervals called check intervals and check whether a channel is being used. The sender node 101, which contains transmission data, confirms that the channel is in an idle mode by performing the LPL operation and transmits a preamble having a length that is longer than the check interval to notify the neighboring receiver node 102 and other nodes 103 that are capable of reception that the sender node 101 contains the transmission data. The sender node 101 then transmits a transmission frame, i.e., the transmission data, after the transmission of the preamble. The neighboring receiver node 102 and other nodes 103 wake up at check intervals to perform the LPL operation. Upon sensing the preamble transmitted by the sender node 101, the neighboring receiver node 102 and other nodes 103 overhear the preamble transmitted by the sender node 101 until they receive a destination address (DA) of the transmission data because the transmission data may be destined for the neighboring receiver node 102 and other nodes 103. If the neighboring receiver node 102 receives the DA of the transmission data and confirms that the neighboring receiver node 102 is a destination of the transmission data, it continues receiving the transmission data to the end of the transmission data. If the other nodes 103 are not the destined objects for the transmission data, they operate in a sleep state. However, according to this conventional technique, both the receiver node 102 and other nodes 103 neighboring the sender node 101 have to unnecessarily receive, i.e. overhear the preamble until they receive the DA of the transmission data. Moreover, a long preamble needs to be used during a transmission process and data has to be overheard (the overhearing is indicated by 104) during a reception process. In other words, the sender node 101, receiver node 102, and other nodes 103 need to consume a large amount of energy.
Second, the B-MAC+ (Marco Avvenuti & Paolo Corsini & Paolo Masci & Alessio Vecchio, ‘Increasing the efficiency of preamble sampling protocols for wireless sensor networks’, ‘Mobile Computing and Wireless Communications International Conference’, September 2006) solves the problems of the B-MAC to some extent.
FIG. 2 is a diagram illustrating operations of sensor nodes, according to the B-MAC+.
Referring to FIG. 2, like in the B-MAC, sender node 211, receiver node 212, and other nodes 213 repeat the LPL operation and a sleep operation.
However, the sender node 211 continuously transmits a short preamble called a wake-up preamble during a preamble interval in order to inform the neighboring receiver node 212 and other nodes 213 of a DA of transmission data and the number of remaining wake-up preambles. In other words, the sender node 211 containing the transmission data continuously transmits the wake-up preamble composed of a preamble-of-wake-up-preamble (PWP) field, a start frame delimiter (SFD) field, a DA field, and a countdown-information-of-wake-up-preamble field after the LPL operation.
The neighboring receiver node 212 and other nodes 213, which wake up from a sleep state at check intervals to perform the LPL operation, wake up upon sensing traffic and receive the wake-up preamble.
In this case, if the neighboring other nodes 213 are not destinations of the transmission data, they check the countdown-information-of-wake-up-preamble field of the received wake-up preamble and sleep until the end of a long preamble. If the neighboring receive node 212 is the destination of the transmission data, it checks the countdown-information-of-wake-up-preamble field of the received wake-up preamble, sleeps until the end of the long preamble, and then wakes up to receive the transmission data.
Therefore, by avoiding the reception of the entire long preamble during a long preamble interval of FIG. 4 corresponding to the preamble interval of FIG. 3, the B-MAC+ solves the overhearing problem of the B-MAC. However, if some of the neighboring other nodes 213 for which the transmission data of the sender node 211 is not destined sleep due to early reception of a wake-up preamble, they may wake up for a next LPL operation during the data transmission of the sender node 211 as indicated by 214. In this case, some of the neighboring other nodes 213 operating as illustrated on a time axis 213a may overhear the wake-up preamble after the LPL operation.